It is known that internal combustion engines in motor vehicles must be started with a starter device, because they cannot start on their own. To that end, typically electrically operated starter motors are used, which are connected to a voltage source via a starter relay, embodied as a so-called engaging relay. At the same time, for starting up, a pinion of the starter motor is brought into engagement with a gear ring, typically mounted on a flywheel of the engine. Once the engine is running on its own, the starter motor has to be disengaged, in order to prevent increased wear and major noise production. If for certain reasons the starter should remain constantly in engagement, then it must be designed for permanent operation even at relatively high rpm.
It is also known that internal combustion engines are typically equipped with a generator that constantly also runs; on the one hand the generator assures a permanent supply for the on-board electrical system and on the other it assures charging of the voltage source necessary for operating the starter motor. This generator is typically driven via the engine crankshaft by means of a v-belt or toothed belt. The generator outputs its rated output typically at a medium rpm, but it must be dimensioned such that even at an idling rpm of the engine, it can produce an adequate electrical output. At the same time, however, it must be designed for the maximum rpm of the engine and must also undergo adequate cooling. Cooling at relatively high rpm is usually done by air ventilation, which is supported by mounted fan blades. This leads to a pronounced undulation in the engine moment and to a mass inertia of the generator, which necessitates a drive via belts.
If the starter motor is to be operated simultaneously as a generator, then the problem first arises of designing it relative to rpm. In operation as a starter motor, a relatively high output at low rpm is needed, because the engine merely has to be started until it reaches its idling rpm. For reliable transmission of such high output, the aforementioned coupling with a pair of gear wheels on the engine side is chosen; this coupling is broken after the engine has started. The generator, however, must be in a position to assure an adequate power output over a wide and higher rpm range, yet without being exposed to the risk of overheating at maximum rpm. If the generator is to be designed for a higher electrical output, however, as is necessary for modern motor vehicles with many electrical consumers, such as control motors and servomotors, then the conventional belt drive rapidly runs up against the limits of its capability.
It is true that embodiments are known which provide simultaneous use of the generator as a starter as well, for instance in the form of a so-called dynastarter in motorcycle and motorbike designs of many years ago. Here, however, the generator had to produce only a very slight output, and therefore it could easily be operated in a range of lesser efficiency after the engine had started. For reasons of space, the generator was often located directly on the end of the crankshaft, where it ran at the engine rpm. The undulating nature of the moment was once again no problem, given the high-speed two-stroke engines typically used.
The older German Patent Application DE 196 29 839.3 also describes an internal combustion engine for motor vehicles, with a gear acting on the drive wheels of the motor vehicle, where a gear input shaft can be coupled to a power takeoff shaft of the engine, and having an electric machine which can be coupled to the gear via an intermediate gear and can be switched over to operate both as a starter motor for starting the engine and as a generator for supplying energy to an on-board electrical system of the motor vehicle. A disadvantage here, however, is once again the necessity of designing it as a starter motor with high torque at low rpm, which is simultaneously unfavorable for operation as a generator at higher rpm.
The object of the invention is therefore to develop a starter-generator machine for an internal combustion engine that can both furnish and reliably transmit adequate mechanical power on engine starting and furnish a sufficiently high electrical output in operation as a generator, when the engine is running at higher rpm.
Accordingly, it is an object of the present invention to provide a starter-generator machine which avoids the disadvantages of the prior art.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in an electrical machine which can be coupled to an engine drive train via a gear in such a way that for synchronizing a main gear, the speed-changing stages in the gear and thus the moment of mass inertia of the electrical machine can be selected in accordance with the desired RPM adaptation of the drive train.
When the electrical machine is designed in accordance with the present invention it offers the advantage that because it is possible to adapt the rpm by means of a gear, the generator can always run in a range of high efficiency when the vehicle engine is running. Even at a low crankshaft rpm, a high electrical power output of the generator is thus assured, and thus the numerous electrical consumers in today""s modern motor vehicles can be reliably supplied with energy, without the risk, for instance if the engine is idling for a long time in a traffic jam, of a slow discharge of the vehicle battery. At the same time, however, the electric machine can furnish the high-torque necessary for starting the engine, if the gear ratio is selected appropriately. For adjusting the rpm of the electric machine, a planetary gear is especially suitable, because of its uncomplicated design and ease of operation. Furthermore, such gears take up only very little space, and so in this case they can easily be integrated in the vehicle.
One advantageous feature arises from an additional idling position in addition to the preferably two variable speed-changing stages of the planetary gear. The disengagement lever can be put in a middle position, as a result of which the operative connection of the electric machine to the engine drive train can be interrupted. After the neutral position has been selected, the rotor of the electric machine slowly runs down. Such a switching position can be especially advantageous to achieve various fuel-saving operating states of the vehicle, such as a mode in which the electrical part of the generator does not also run. Since conventional generators are permanently driven at a constant gear ratio, they consume a certain quantity of additional fuel even when the consumers are turned off and the battery is completely charged. An additionally provided idling position in the planetary gear conversely allows a shutoff of the generator if the battery is charged and an interruption in the operative connection between the generator and the drive train. Such a switching position is also advantageous to protect the electric machine against an overload or overheating. For example at excessively high rpm or overly severe heating of the electric machine, the neutral position can be selected automatically, and as a result an effective overload protection can be achieved.
It is also advantageous in the machine of the invention that only one switching position of the planetary gear at a given time is possible. Compared with conventional versions with a magnetic clutch and a freewheel, the apparatus of the invention has the advantage that unintentional incorrect shifting, that is, selecting two gears at once, is possible. Furthermore, the gear wheels of the planetary gear are permanently in engagement with one another; that is, the switchover of the gear proceeds without disengagement of the gear wheels, so that additional synchronizing means can be dispensed with.
The machine of the invention also has the advantage of requiring only extremely slight additional energy. Since the selected switching position of the planetary gear at a given time and thus the gear ratio remain constant without any further energy being supplied from outside, an additional energy supply is necessary only at the moment of switchover of the gear ratio. This switchover energy can be brought to bear mechanically. However, electromagnetically, hydraulically or pneumatically actuated switchover devices are equally possible.
Because the starter and generator are spatially combined into a single component, the effort of construction and installation are also markedly reduced, which on the one hand has a cost advantage. Since the space required in the engine is also reduced, there is increased engineering freedom in positioning the engine in the vehicle along with its accessory systems. The conventional production systems and the know-how for producing starters and generators can continue to be used. Various components correspond unchanged to the present series of starters and generators.
It is also advantageous if the starter-generator machine of the invention is liquid-cooled which can be done by connecting it to the water cooling loop of the engine. Along with the embodiment shown with water cooling, an air- cooled version with its own or an additional cooling fan can also be realized.
Finally, it is advantageous if the starter-generator machine and in particular the switchable planetary gear are oil-lubricated, which can expediently be done by supplying them with the transmission oil of the shift transmission.
A further advantageous possibility for using the electric machine resides in reinforcing the gear synchronization. This can be done by means of targeted torque surges for acceleration or braking of the gear input shaft operatively connected to the electric machine. In this way, given a suitable dimensioned electric machine, the conventional synchronization in the shift transmission can be reinforced or can even be omitted entirely. However, that requires a marked oversize, since for additionally accelerating the gear input shaft at higher engine rpm, a considerable mechanical output must be brought to bear.
Further advantageous features of the invention will become apparent from the other characteristics, recited in the dependent claims.